The present invention relates to a method for diagnosing an SCR catalyst system of an internal combustion engine, wherein the SCR catalyst system comprises at least one first SCR catalyst device and at least one second SCR catalyst device. The invention furthermore relates to an SCR catalyst system which is designed to carry out the diagnostic method, and to a computer program, a machine-readable storage medium and an electronic control unit, which are provided for carrying out the diagnostic method.
In order to comply with the legally prescribed exhaust gas limits during the operation of motor vehicles, complex exhaust gas aftertreatment systems are used. In the context of “onboard diagnosis” (OBD), stricter laws in the field of diagnosing components relevant to emissions require monitoring of all the exhaust gas aftertreatment components and of the sensors used in respect of compliance with the OBD limits, which are generally specified as a multiple of the legally defined emissions limit.
SCR catalysts (Selective Catalytic Reduction) for reducing nitrogen oxides (NOx) are known. The basic principle of the SCR catalyst consists in that nitrogen oxide molecules are reduced to elementary nitrogen on the catalyst surface in the presence of ammonia (NH3) as a reducing agent. The required reducing agent, e.g. in the form of an aqueous urea solution, is injected into the exhaust line upstream of the SCR catalyst in a manner dependent on requirements. The required metering quantity is generally determined in an electronic control unit, in which optimized strategies for the operation and monitoring of the SCR system are stored.
Currently known SCR catalysts store NH3 on the catalyst surface. The NH3 stored on the catalyst surface is described as the NH3 filling level. DE 10 2007 040 439 A1 describes a monitoring strategy in which the NH3 storage capacity of the SCR catalyst is analyzed as an indicator of aging or damage of the catalyst, wherein a phase involving super stoichiometric reducing agent metering (overmetering) and an emptying test are carried out and the NH3 storage capacity is inferred from characteristic values which represent the nitrogen oxide conversion rate. DE 10 2012 201 749 A1 discloses a modification of this method for diagnosing storage capacity, wherein conditioning of the SCR catalyst is carried out before the start of diagnosis or the overmetering phase in order to establish an operating point in the form of a particular NH3 filling level as a starting point for diagnosis.
To achieve higher conversion rates during nitrogen oxide reduction in the exhaust line, there are already known systems which comprise two separate SCR catalyst devices arranged in series. German Laid-Open Applications DE 10 2013 203 580 A1 and DE 10 2013 203 578 A1 describe monitoring methods for exhaust gas aftertreatment systems having two SCR catalyst devices.